Spinal fusion treatment is considered a standard of care for intractable lower back pain arising from degenerative disc disease and/or spinal instability. Fusion includes immobilizing the painful spine segments and encouraging bone growth across the immobilized level. In the cervical spine, anterior decompression and fusion is the gold standard.
Spine fusion was first performed without instrumentation using bone grafts, the bone grafts often being obtained from the patient's own body (i.e., from the iliac crest). Instrumented fusion, using rods, plates, and screws, was initially developed to provide rigid stability to the spine while the implanted bone grafts fused across the treated level. Since then, fusion implants have become common, replacing bone grafts.
Conventional implants are designed to facilitate primarily through-growth, or fusion resulting from growth of bone through holes or channels through the implants, for example in order to reach other bone. For example, Medtronic LT Cages® are thimble-like titanium devices that are packed with a collagen sponge soaked in rhBMP-2 (recombinant human bone morphogenic protein 2). A pair of the cages are inserted between adjacent vertebrae to initiate bone growth through the cages. Conventional CFR-PEEK cages (carbon fiber reinforced PEEK plastic cages) also rely upon through-growth—for example, the Jaguar™ and Saber™ Lumbar I/F CAGE Systems house autologous cancellous bone grafts that grow through the cages to join with adjacent vertebrae. Alphatec Novel TL spacers are made of PEEK plastic and include an internal chamber allowing for growth of bone therein.
Although effective, through-growth occurs slowly, for example, over a period of a year or more. Through-growth can be further delayed if the implant area is not immobilized. Even micro-motion of the implant area can disturb and disrupt bone growth, leading to increased incidence of subsidence and pseudarthrosis.
Some conventional devices attempt to improve implant stabilization by encouraging bone on-growth—a comparatively rapid, planar growth of bone upon surfaces of an adjacent implant, or upon surfaces of adjacent bone. For example, on-growth may be encouraged by coating a titanium cage with a chemical such as hydroxyapatite, a mineral naturally found in bone, to encourage new-grown bone to stick to the implant surface (for example, as is done with titanium dental implants). However, because they are radio-opaque, titanium cages and implants may hinder diagnostic assessment of bone growth, whether coated with hydroxyapatite or not. For example, implants made primarily of radio-opaque titanium may obscure visualization of bone growth (e.g., through-growth) on x-rays. Titanium may likewise cause signal artifact with MRIs or CTs, making it difficult to determine if fusion has occurred.
In order to avoid the visualization problems of titanium implants, attempts have been made to mix hydroxyapatite with, or apply hydroxyapatite to, radiolucent PEEK plastic (or other non-scattering biocompatible material, e.g., HDPE) to form a cage/implant. However, hydroxyapatite content embrittles the material and weakens such implants. In addition, PEEK provides poorer fixation than titanium, and thus, PEEK implants must often be supplemented with posterior pedicle screw and rod instrumentation.